The answer is simple: Fixing your cat will give him or her better health and maybe even a longer life.
Bob Barker always reminded his audience to spay or neuter their pets before signing off of The Price is Right. He was a huge advocate for controlling the pet population. This is why I started fixing my cats in the first place. I didn't want my cats mating, adding litters of unwanted kittens to the world. Too many pets are homeless, and so many of them will never find a loving owner. For me, this was a good enough reason to always sterilize my pets.
Others, however, don't feel the need to fix their cats. Why should Fluffy have her girl parts removed if she is an only kitty? She always stays indoors, so there's no way that a boy cat could ever mate with her. Spaying is expensive, and Fluffy will have to go under general anesthesia.
To those people, I have to remind them that fixing your cats not only prevents unwanted pregnancies, but it also prevents health issues later on in life.
One of the biggest illnesses early sterilization prevents is cancer. Male cats that are fixed cannot get testicular cancer as they age. When a female is spayed, you are eliminating her chance of developing uterine or ovarian cancer. On top of that, her risk of having breast cancer is greatly decreased. These cancers are very difficult to treat in animals, so it's best to eliminate the source.
Cats that are fixed also have better temperaments. Both female and male cats become calmer. This reduces the likelihood of them instigating fights with other animals. Many untreatable diseases, such as feline AIDS and feline leukemia, are caught from fighting injuries. After watching a kitty die from feline AIDS, I know how painful and devastating this disease can be.
If you're still thinking about the pain your cat will endure during the procedure, consider this: Your cat will be asleep during the entire surgery. They will not feel a thing. After they wake up, your vet will prescribe them painkillers to reduce any discomfort. Most animals are back to their normal selves within a few days.
Give your kitty a better life, and have them sterilized while they are still young.